Method for manufacturing plastic worked article

ABSTRACT

The method for manufacturing a plastic worked article includes the steps of shaping an article for plastic working by a semisolid molding in which molten metal including both a solid phase and a liquid phase is introduced into a die; performing a pre-plastic working heat treatment on the article for plastic working for causing blisters by expanding gas cavities included in the article for plastic working; and forming a plastic worked article by plastic working the article for plastic working after the pre-plastic working heat treatment. The plastic working of the article for plastic working is performed in such a manner that a tensile stress applied direction at a tensile stress applied portion to which tensile stress is principally applied in actual use of the plastic worked article substantially accords with a plastic flow direction at a portion of the article for plastic working corresponding to the tensile stress applied portion.

[0001] This is a continuation of Application PCT/JP02/02532, filed Mar. 18, 2002, now abandoned.

FIELD OF THE INVENTION

[0002] The present invention relates to a manufacturing method for a plastic worked article in which an article for plastic working shaped by a semisolid molding is plastic worked after causing blisters by expanding gas cavities included in the article for plastic working through a pre-plastic working heat treatment.

BACKGROUND OF THE INVENTION

[0003] Cast-forging in which an article for forging having been shaped into a near-net shape by casting is forged into a forged article, that is, a final product, is a process capable of providing a forged article with mechanical properties equivalent to those obtained by casting at comparatively low cost. On the other hand, the mechanical properties of a forged article are generally improved by a T6 heat treatment (a solution treatment followed by an ageing precipitation hardening treatment). In the case where the T6 heat treatment is applied to a forged article obtained by the cast-forging, however, blisters derived from gas cavities, that is, internal defects introduced during the forging, are caused on the surface thereof, which degrades the mechanical properties and the appearance quality of the forged article.

[0004] As a countermeasure, Japanese Laid-Open Patent Publication No. 2000-197956 discloses the following method: An article for forging is shaped by filling a cavity in a die with molten light metal, the obtained article for forging is subjected to a heat treatment for causing blisters derived from expansion of an internal gas, and the resultant article for forging having been heat treated is forged. According to this publication, when blisters are thus caused at an article stage, the blisters can be ruptured by forging, so that blisters can be definitely prevented from being caused in an ultimately obtained forged article.

[0005] When the heat treatment is carried out before the forging as disclosed in Japanese Laid-Open Patent Publication No. 2000-197956, however, anisotropy in the mechanical properties is caused in the ultimately obtained forged article, and hence, the tensile strength is higher in a plastic flow direction than in a forging direction. This is probably for the following reason: An article for forging produced by the semisolid molding has a microstructure in which a matrix of a liquid phase part formed by hardening a liquid phase is interspersed with solid phase parts that were solid in a semisolid state, and there remains an interface between the liquid phase part and the solid phase parts after the heat treatment performed before the forging. When this article is forged, the solid phase parts are compressed in the forging direction as well as elongated in the plastic flow direction substantially perpendicular to the forging direction. As a result, interfacial flaking between the liquid phase part and the solid phase parts can be easily caused in the forging direction than in the plastic flow direction.

SUMMARY OF THE INVENTION

[0006] An object of the invention is providing a method for manufacturing a plastic worked article in consideration of anisotropy in the mechanical properties caused in a plastic worked article manufactured by plastic working an article for plastic working shaped by the semisolid molding and having been heat treated before the plastic working.

[0007] In order to achieve the object, an article for plastic working is plastic worked, in consideration of the anisotropy in the mechanical property caused in a plastic worked article, in such a manner that a tensile stress applied direction at a tensile stress applied portion to which tensile stress is principally applied in actual use of the plastic worked article substantially accords with a plastic flow direction at a portion of the article for plastic working corresponding to the tensile stress applied portion.

[0008] Specifically, the present invention is a method for manufacturing a plastic worked article by plastic working an article for plastic working, including the steps of shaping an article for plastic working by a semisolid molding in which molten metal including both a solid phase and a liquid phase is introduced into a die; performing a pre-plastic working heat treatment on the article for plastic working for causing blisters by expanding gas cavities included in the article for plastic working; and forming a plastic worked article by plastic working the article for plastic working after the pre-plastic working heat treatment, and the plastic working of the article for plastic working is performed in such a manner that a tensile stress applied direction at a tensile stress applied portion to which tensile stress is principally applied in actual use of the plastic worked article substantially accords with a plastic flow direction at a portion of the article for plastic working corresponding to the tensile stress applied portion.

[0009] According to this manufacturing method, the plastic working of the article for plastic working is performed in such a manner that the tensile stress applied direction at the tensile stress applied portion to which tensile stress is principally applied in actual use of the plastic worked article substantially accords with the plastic flow direction at the portion of the article for plastic working corresponding to the tensile stress applied portion. Therefore, even when the anisotropy in the mechanical properties occurs in the plastic worked article obtained by plastic working the article for plastic working having been subjected to the pre-plastic working heat treatment, the reinforcing effect of the plastic working can be effective exhibited in the resultant plastic worked article. Also, even when the plastic worked article has a smaller thickness, necessary strength can be attained, so as to lower the cost.

[0010] The plastic working is not particularly specified and may be, for example, forging, spinning or the like.

[0011] The tensile stress applied portion is the portion to which the tensile stress is principally applied in actual use of the plastic worked article, and specifically corresponds to a portion where the integrated value of the tensile stress is relatively large.

[0012] Also, the semisolid molding is a shaping method in which molten metal in a semisolid state including both a solid phase and a liquid phase of a raw material metal is introduced into a die, and the semisolid state is generally obtained by beating the raw material metal below the melting point. In the resultant article for plastic working, a microstructure in which a matrix of a liquid phase part formed by hardening the liquid phase is interspersed with solid phase parts that were solid in the semisolid state is formed.

[0013] The pre-plastic working heat treatment is performed in order to cause blisters, which are derived from expansion of gas cavities, that is, internal defects such as gas defects included in the vicinity of the surface of the article for plastic working, on and/or in the vicinity of the surface thereof, and the blisters are ruptured by the plastic working. As a result, the number of gas cavities included in the plastic worked article is reduced, whereby improving the mechanical properties of the plastic worked article. Accordingly, in using an article for plastic working shaped by a shaping method where gas cavities are more easily formed, such as the semisolid molding using an injection molding apparatus equipped with a cylinder and a screw, the effect to improve the mechanical properties of the plastic worked article can be particularly remarkably exhibited.

[0014] The pre-plastic working heat treatment is preferably carried out with a processing temperature in a range of 300° C. to 550° C. Thus, the blisters can be definitely caused on and/or in the vicinity of the surface of the article for plastic working while suppressing oxidation caused by the heat treatment. Specifically, when the processing temperature of the pre-plastic working heat treatment is lower than 300° C., the blisters may be insufficiently caused on and/or in the vicinity of the surface of the article for plastic working. Alternatively, when the heat treatment is carried out at a temperature exceeding 550° C., the surface may be largely oxidized or partially eutectic fused. In consideration of these, the processing temperature is more preferably in a range of 350° C. to 450° C.

[0015] Furthermore, the pre-plastic working heat treatment is preferably carried out with a processing time of 1 hour or more. Thus, the blisters can be effectively caused on the surface of the article for plastic working. In addition, the microstructure composed of the liquid phase part and the solid phase parts is homogenized as in the solution treatment of the T6 heat treatment, so that the anisotropy in the mechanical properties caused by the plastic working can be reduced. In the case where the pre-plastic working heat treatment is carried out at a processing temperature in a range of 350° C. to 450° C., the processing time is preferably 20 hours or less and more preferably in a range of 10 hours to 20 hours.

[0016] When the pre-plastic working heat treatment is performed at a comparatively high temperature for a comparatively long time so as to cause the blisters by expanding the gas cavities included in the article for plastic working, a post-plastic working heat treatment, if performed, may be performed at a comparatively low temperature for a comparatively short time. Thus, occurrence of blisters can be suppressed in the post-plastic working heat treatment.

[0017] The post-plastic working heat treatment is preferably performed with a processing temperature in a range of 250° C. to 400° C. and a processing time in a range of 20 minutes to 10 hours. Thus, the post-plastic working heat treatment is performed after the plastic working at a higher temperature for a shorter time than in the ageing precipitation hardening treatment of the T6 heat treatment, and hence, ductility can be effectively improved while retaining the tensile strength and the yield strength of the plastic worked article as obvious from an experiment described below. The processing temperature is in a range of 250° C. to 400° C. because when the processing temperature is lower than 250° C., the ductility cannot be sufficiently improved and when it is higher than 400° C., the yield strength is largely lowered. Furthermore, the processing time is in a range of 20 minutes to 10 hours because when the processing time is shorter than 20 minutes, the ductility cannot be sufficiently improved and when it is longer than 10 hours, the ductility may be lower than that attained without the heat treatment. Accordingly, the processing time is preferably 5 hours or less and most preferably 1 hour.

[0018] The plastic worked article manufactured in the aforementioned manner is not particularly specified but may be, for example, a road wheel, an engine bracket or the like.

[0019] When the plastic worked article is a road wheel, the plastic working of the article for plastic working is performed in such a manner that a direction substantially along an extending direction of a spoke, namely, the tensile stress applied direction, of a wheel disk corresponding to the tensile stress applied portion of the road wheel substantially accords with a plastic flow direction at a portion of the article for plastic working corresponding to the wheel disk. Thus, the strength along the extending directions of the spokes of the wheel disk can be effectively improved. Alternatively, the plastic working of the article for plastic working is performed in such a manner that a direction substantially along a width of a wheel rim of the road wheel, namely, the tensile stress applied direction at the tensile stress applied portion, substantially accords with a plastic flow direction at a portion of the article for plastic working corresponding to the wheel rim. Thus, the strength along the width of the wheel rim can be effectively improved.

[0020] In a specific aspect of the plastic working, a thick part is provided on the portion of the article for plastic working corresponding to the tensile stress applied portion, and the article for plastic working is shaped into a desired shape by the plastic working by allowing the thick part to flow in a direction substantially according with the tensile stress applied direction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a partial cross-sectional view of an injection molding apparatus according to an embodiment of the invention;

[0022]FIG. 2A is a front view of a wheel disk of a road wheel exemplified as a forged article of the embodiment and FIG. 2B is a cross-sectional view taken on line IIB-IIB of FIG. 2A;

[0023]FIG. 3A is a vertical cross-sectional view of a spoke corresponding portion of an article for forging used for shaping the road wheel exemplified as the forged article of the embodiment and FIG. 3B is a vertical cross-sectional view of a spoke of the road wheel obtained by forging the article for forging;

[0024]FIG. 4A is a vertical cross-sectional view of a spoke corresponding portion of an article for forging of a first example and a resultant spoke obtained by forging, FIG. 4B is a vertical cross-sectional view of a spoke corresponding portion of an article for forging of a second example and a resultant spoke obtained by forging, and FIG. 4C is a vertical cross-sectional view of a spoke corresponding portion of an article for forging of a third example and a resultant spoke obtained by forging;

[0025]FIG. 5 is a vertical cross-sectional view of a road wheel exemplified as a forged article of another embodiment of the invention;

[0026]FIG. 6A is a vertical cross-sectional view of an article for forging used for shaping a road wheel exemplified as a forged article of another embodiment of the invention and FIG. 6B is a vertical cross-sectional view of the road wheel obtained by forging the article for forging;

[0027]FIGS. 7A and 7B are explanatory diagrams of states attained before and after forging in Experiment 1;

[0028]FIG. 8 is a graph for showing the 0.2% yield strength, the tensile strength and the elongation of tensile test samples of a forged article produced under conditions A in Experiment 1;

[0029]FIG. 9 is a graph for showing the 0.2% yield strength, the tensile strength and the elongation of tensile test samples of a forged article produced under conditions B in Experiment 1;

[0030]FIGS. 10A and 10B are drawings of microstructures on surfaces of the forged articles produced under the conditions A and B in Experiment 1;

[0031]FIG. 11 is a graph for showing the relationship, obtained in a forged article made from an alloy B, between the processing temperature of a post-forging heat treatment carried out in Experiment 2 and the 0.2% yield strength, the tensile strength and the elongation after fracture;

[0032]FIG. 12 is a graph for showing the relationship, obtained in a forged article made from an alloy C, between the processing temperature of a post-forging heat treatment carried out in Experiment 2 and the 0.2% yield strength, the tensile strength and the elongation after fracture;

[0033]FIGS. 13A, 13B, 13C and 13D are drawings of microstructures on surfaces of forged articles made from the alloy B having been subjected to the post-forging heat treatment under various conditions in Experiment 2;

[0034]FIGS. 14A, 14B, 14C and 14D are drawings of microstructures on surfaces of forged articles made from the alloy C having been subjected to the post-forging heat treatment under various conditions in Experiment 2;

[0035]FIG. 15 is a graph for showing the relationship, obtained in a forged article made from the alloy B, between the processing time of a post-forging heat treatment carried out in Experiment 3 and the 0.2% yield strength, the tensile strength and the elongation after fracture; and

[0036]FIG. 16 is a graph for showing the relationship, obtained in a forged article made from the alloy C, between the processing time of a post-forging heat treatment carried out in Experiment 3 and the 0.2% yield strength, the tensile strength and the elongation after fracture.

DETAILED DESCRIPTION OF THE INVENTION

[0037] A method for manufacturing a forged article according to a preferred embodiment of the invention will now be described.

[0038] (Forging Process—Injection Molding Process)

[0039] <Injection Molding Apparatus>

[0040]FIG. 1 shows an injection molding apparatus 1 for shaping an article for forging (an article for plastic working) of light metal alloy used in this embodiment.

[0041] The injection molding apparatus 1 includes a main body 2, a screw 3 rotatably supported by the main body 2, a rotation driving unit 4 disposed behind the main body 2 for rotationally driving the screw 3, a cylinder 5 fixed on the main body 2 so as to surround the screw 3, heaters 6 arranged around the circumference of the cylinder 5 at a predetermined pitch along the lengthwise direction of the cylinder 5, a hopper 7 for storing a light metal alloy raw material introduced therein, a feeder 8 for measuring the material stored in the hopper 7 and supplying the material into the injection molding apparatus 1, and a die 9 attached on the end of the cylinder 5.

[0042] An injection mechanism for propelling the screw 3 along the lengthwise direction within the cylinder 5 is provided on the main body 2. When the injection mechanism detects that the screw 3 has retracted a preset distance due to the force of molten light metal alloy transported forward, it stops the rotation and retraction of the screw 3, and propels the screw 3 forward with a predetermined timing to inject molten metal. The speed at which the screw 3 is propelled forward is controllable, so as to control the speed at which the molten metal is introduced into a cavity 12 in the die 9.

[0043] A nozzle 10 is provided at the end of the cylinder 5, so that molten metal having been stirred and kneaded within the cylinder 5 can be injected into the cavity 12 through the nozzle 10. The molten metal is injected into the cavity 12 when a predetermined amount of molten metal is gathered in a front portion of the cylinder 5, and hence, it is necessary to prevent the molten metal from flowing out through the nozzle 10 until the sufficient amount for the injection is gathered. For this purpose, the temperature of the nozzle 10 is controlled as follows: While the molten metal is gathering in the front portion of the cylinder 5, the nozzle 10 is clogged with a cold plug made from solidified molten metal, and when the molten metal is to be injected, the cold plug is easily removed and pressed out toward the die 9 together with the injected molten metal.

[0044] The heaters 6 provided around the circumference of the cylinder 5 are divided into a plurality of zones in each of which the temperature is controlled, so that the temperature increases along the lengthwise direction of the cylinder 5 toward the front portion thereof Thus, the temperature of the light metal alloy raw material is increased while it is being transported within the cylinder 5 by the screw 3, so that the light metal alloy raw material can become molten metal in a semisolid state below the melting point when it reaches the front portion of the cylinder 5.

[0045] The hopper 7, the feeder 8, the cylinder 5 and passages joining them are filled with an inert gas (such as an Ar gas) for preventing oxidation of the light metal alloy.

[0046] The die 9 has a runner 11 for guiding the molten metal injected from the nozzle 10. The runner 11 extends straight from the nozzle 10 of the cylinder 5 and then rises vertically to form an L-shape. A plug receptacle 11 a is provided at the corner of the L-shape for receiving the cold plug having been removed from the nozzle 10. Also, the die 9 includes a cavity 12 connected to the runner 11, a gate 13 serving as the boundary between the cavity 12 and the runner 11, and an overflow 14 positioned away from the gate 13 of the cavity 12 for accepting a gas having been contained in the cavity 12 and displaced by the molten metal.

[0047] <Injection Molding Method>

[0048] The injection molding method employed for an article for forging of light metal alloy will now be described.

[0049] First, chips of light metal alloy (such as Mg—Al alloy) are placed in the hopper 7 of the injection molding apparatus 1 as a raw material. A predetermined weight of the light metal alloy chips is measured and supplied to the injection molding apparatus 1 by the feeder 8.

[0050] Thereafter, the light metal alloy chips are transported by the rotation of the screw 3 into the cylinder 5 that is heated, and are sufficiently stirred and kneaded by the rotation of the screw 3 and heated to a predetermined temperature within the cylinder 5. As a result, the light metal alloy chips are changed into semisolid light metal alloy below the melting point.

[0051] The thus obtained molten metal alloy is pushed forward by the screw 3 to be gathered in the front portion of the cylinder 5, and the screw 3 retracts due to the pressure of the gathered molten metal alloy. At this point, the temperature of the nozzle 10 provided on the cylinder 5 is lowered so as to clog the nozzle 10 with a cold plug formed by solidifying some of the molten metal. Thus, the molten metal is prevented from flowing through the nozzle 10 out of the cylinder 5.

[0052] When the screw 3 retracts by a predetermined distance, the injecting mechanism of the main body 2 detects the retraction so as to stop the rotation and the retraction of the screw 3. At this point, the molten metal in an amount for a single injection is gathered in the front portion of the cylinder 5.

[0053] Then, the screw 3 is pushed forward by the injection mechanism to apply a pressure to the molten metal. As a result, the molten metal pushes out the cold plug toward the die 9 and is injected through the nozzle 10 into the cavity 12. At this point, the semisolid light metal alloy enters the cavity as a laminar flow or near-laminar flow, and hence, air is minimally trapped, resulting in shaping an article for forging with few gas defects such as gas cavities and shrinkage cavities. The removed cold plug is caught by the plug receptacle 11 a of the runner 11.

[0054] Finally, after the molten metal is solidified, the die 9 is opened and the obtained injection molded material to be used as the article for forging is taken out.

[0055] The thus obtained article for forging has a microstructure in which a matrix of a liquid phase part formed by hardening a liquid phase is interspersed with solid phase parts that were solid in the semisolid state.

[0056] (Pre-Forging Heat Treatment)

[0057] The article for forging injection molded in the aforementioned manner is subjected to a pre-forging heat treatment (a pre-plastic working heat treatment) carried out with a processing temperature in a range of 300° C. to 550° C. and a processing time of 1 hour or more (preferably with a processing temperature in a range of 350° C. to 450° C. and a processing time in a range of 1 to 20 hours, and more preferably with a processing temperature in a range of 350° C. to 450° C. and a processing time in a range of 10 to 20 hours). Through this pre-forging heat treatment, the microstructure of the article for forging is homogenized (not completely but) to an extent that the boundaries between the liquid phase part and the solid phase parts become difficult to identify. Also, the gas cavities, that is, the internal defects in the vicinity of the surface of the article for forging, are expanded to cause blisters on and in the vicinity of the surface of the article for forging.

[0058] (Forging Process)

[0059] The article for forging having been subjected to the pre-forging heat treatment is then subjected to either fully enclosed die forging (fully enclosed die plastic working) or non-fully enclosed die forging (non-fully enclosed die plastic working). The fully enclosed die forging is performed in a forging die whose forging space is completely closed, while the non-fully enclosed die forging is performed in a forging die where at least part of the article for forging is not inhibited and so is free to deform plastically. At this point, the article for forging is forged in such a manner that a tensile stress applied direction at a tensile stress applied portion to which tensile stress is principally applied in actual use of the resultant forged article substantially accords with a plastic flow direction at a portion of the article for forging corresponding to the tensile stress applied portion.

[0060] This will be specifically described by exemplifying a road wheel. In actual use of a wheel disk 16 of a road wheel 15, as shown in FIGS. 2A and 2B, tensile stress is principally applied to spokes 16 a along directions shown with arrows in the drawings (namely, directions substantially along extension of the spokes). In other words, the spokes 16 a correspond to the tensile stress applied portion. Therefore, an article for forging 17 having one or more thick parts on the back face of a spoke corresponding portion 17 a as shown in FIG. 3A is shaped by casting, and as shown in FIG. 3B, the thick part of the spoke corresponding portion 17 a is made to plastic flow along directions shown with arrows in the drawing so that the plastic flow directions can substantially accord with the tensile stress applied direction in actual use of the spoke 16 a, namely, the direction substantially along the extension of the spoke. Strictly speaking, tensile stress is applied onto the front face of the spoke 16 a with compression stress applied onto the back face thereof, and hence, it is desired to provide the forging strongly on the front face of the spoke 16 a. When the forging is provided strongly on the front face of the spoke 16 a, however, the surface property of the front face attained after the forging is degraded. Therefore, the material for the article, the dimension of the article 17 for forging, the finished dimension of the road wheel 15 are properly set so that the reinforcing effect of the forging can be exhibited on the front face even when the forging is performed from the back face. Specific examples are shown in FIGS. 4A through 4C. In a first example shown in FIG. 4A, a spoke 16 a having a uniform thickness is shaped by forging an article 17 for forging having a thick part rising at the center in the radial direction on the back face thereof similarly to that shown in FIGS. 3A and 3B. In a second example shown in FIG. 4B, a spoke 16 a having a uniform thickness is shaped by forging an article 17 for forging having wavy irregularities formed in the radial direction on the back face thereof with projected portions serving as thick parts. In a third example shown in FIG. 4C, a spoke 16 a with a smaller thickness at the center and a larger thickness at the ends thereof is shaped by forging an article 17 for forging having a thick part rising at the center in the radial direction on the back face thereof similarly to that shown in FIG. 4A. In any of these examples, when the dimensions and shapes of the article 17 for forging and the spoke 16 a are properly set, forging enters a portion closer to the front face beyond the center line along the thickness direction of the spoke 16 a to be shaped as shown with arrows in each drawing even when the forging is performed from the back face, so that the reinforcing effect can be exhibited on the front face.

[0061] During the forging, some of the blisters caused in the pre-forging heat treatment on and in the vicinity of the surface of the article for forging are ruptured. As a result, the number of internal defects can be reduced as compared with that in the article for forging obtained after the injection molding.

[0062] (Post-Forging Heat Treatment)

[0063] The forged article having been shaped by the forging process is then subjected to a post-forging heat treatment (a post-plastic working heat treatment) carried out with a processing temperature in a range of 250° C. to 400° C. and a processing time in a range of 20 minutes to 10 hours, and the resultant is cooled to obtain a final forged article. Since the pre-forging heat treatment is carried out at a comparatively high temperature for a comparatively long time, the post-forging heat treatment can be carried out at a comparatively low temperature for a comparatively short time. In addition, since the number of internal defects is reduced through the forging process, occurrence of blisters can be suppressed in the post-forging heat treatment.

[0064] According to the aforementioned method for manufacturing a forged article, an article for forging is forged in such a manner that a tensile stress applied direction at a tensile stress applied portion to which tensile stress is principally applied in actual use of the resultant forged article substantially accords with a plastic flow direction at a portion of the article for forging corresponding to the tensile stress applied portion. Therefore, even when the anisotropy in the mechanical properties is caused in the forged article to be manufactured by forging the article for forging after a pre-forging heat treatment, the reinforcing effect attained by the forging can be effectively exhibited in the final forged article to be manufactured. Also, even when the forged article has a smaller thickness, the necessary strength can be attained, and hence, the cost can be lowered.

[0065] Furthermore, since the pre-forging heat treatment is carried out with a processing temperature in a range of 300° C. to 500° C. (preferably 350° C. to 450° C.) and a processing time of 1 hour or more (preferably in a range of 1 to 20 hours and more preferably 10 to 20 hours), blisters can be definitely caused due to expansion of gas cavities formed on and in the vicinity of the surface of the article for forging. Since these blisters are ruptured through the forging, the number of gas cavities included in the resultant forged article can be reduced and the mechanical properties of the forged article can be improved. In addition, the microstructure composed of the liquid phase part and the solid phase parts is homogenized, resulting in reducing the anisotropy in the mechanical properties of the article for forging.

[0066] Moreover, the post-forging heat treatment is carried out after the forging process at a higher temperature for a shorter time than in the ageing precipitation hardening treatment of the T6 heat treatment. Therefore, while retaining the tensile strength and the yield strength of the forged article, the ductility can be effectively improved.

[0067] Alternative Embodiments

[0068] Although the article for forging is shaped by the injection molding in the aforementioned embodiment, the method for shaping the article for forging is not limited to this but may be any of other shaping methods such as a die-cast method.

[0069] Although the article for forging is made from a light metal (such as Mg—Al alloy) in the embodiment, the material for the article for forging is not limited to this but may be another metal material.

[0070] Although the plastic working is performed by forging in the embodiment, the plastic working is not limited to this but may be spinning or the like.

[0071] Furthermore, although the road wheel 16 is described as a specific example of the forged article in the embodiment, the forged article is not limited to this but may be an engine bracket or the like.

[0072] Although the wheel disk 16 is plastic worked by forging in the embodiment, a wheel rim 18 may be plastic worked by spinning as follows: As shown in FIG. 5, in actual use of the wheel rim 18 of the road wheel 15, tensile stress is principally applied in the wheel rim 18 along directions shown with arrows in the drawing (namely, directions substantially along the width of the wheel rim 18). In other words, the wheel rim 18 corresponds to the tensile stress applied portion. Therefore, through the spinning of a rim corresponding portion 19 a of an article 19 for forging as shown in FIG. 6A, the plastic flows are caused in directions shown with arrows in FIG. 6B so that the plastic flow direction can substantially accord with the tensile stress applied direction in actual use of the wheel rim 18, namely, the direction substantially along the width of the wheel rim 18. As a result, even when the wheel rim 18 has a smaller thickness, the necessary strength can be attained.

[0073] (Experiment 1)

[0074] <Experiment Method>

[0075] Two articles for forging each in the form of a metal block were shaped by using the injection molding apparatus from an alloy A having a composition listed in Table 1.

[0076] One of the article for forging was subjected to a forging process and a heat treatment under conditions A listed in Table 2. Specifically, the article for forging was subjected to a pre-forging heat treatment at 400° C. for 10 hours, cooled and then subjected to non-fully enclosed die forging to give a forged article in the shape of a plate. Thereafter, the forged article was subjected to a post-forging beat treatment at 350° C. for 1 hour and cooled to give a final forged article. In this case, the non-fully enclosed die forging was performed to attain a forging working rate of 50%. Specifically, as shown in FIG. 7, the non-fully enclosed die forging was performed so as to attain (h₀-h₁)/h₀×100=50 (the forging working rate), wherein h₀ is an initial thickness along the forging direction and h₁ is a thickness obtained after the forging.

[0077] The other article for forging was subjected to a forging process and a heat treatment under conditions B listed in Table 2. Specifically, the article for forging was subjected to non-fully enclosed die forging to give a forged article. The resultant was subjected to a solution treatment at 400° C. for 10 hours and cooled, and then subjected to an ageing precipitation hardening treatment at 175° C. for 16 hours and cooled (namely, subjected to the T6 heat treatment) to give a final forged article in the form of a plate. In this case, the non-fully enclosed die forging was performed to attain a forging working rate of 50%.

[0078] From each of the thus obtained forged articles, a tensile test sample 21 in which the tensile direction accords with the forging direction and a tensile test sample 22 in which the tensile direction accords with the plastic flow direction perpendicular to the forging direction were cut out as shown in FIG. 7B.

[0079] A tensile test was carried out on the tensile test sample 21 having the tensile direction according with the forging direction and the tensile test sample 22 having the tensile direction according with the plastic flow direction cut out from each of the forged articles.

[0080] Also, a microstructure on the surface of each forged article was observed with a microscope. TABLE 1 Al Zn Mn Si Cu Ni Fe Mg Alloy A 7.1 0.66 0.24 — 0.002 <0.001 0.002 remainder Alloy B 9.1 0.78 0.24 — 0.001 0.0009 0.003 remainder Alloy C 6.9 0.60 0.23 0.01 0.002 0.0005 0.002 remainder

[0081] TABLE 2 Condition A Heat treatment at 400° C. for 10 hrs. → Air cooling → Hot forging → Heat treatment at 350° C. for 1 hr. → Air cooling Condition B Hot forging → Heat treatment at 400° C. for 10 hrs. → Air cooling → Heat treatment at 175° C. for 16 hrs. → Air cooling

[0082] <Experiment Results>

[0083]FIG. 8 shows the results of the tensile test (0.2% yield strength, the tensile strength and the elongation after fracture) obtained in each tensile test sample of the forged article manufactured under the conditions A. FIG. 9 shows the relationship corresponding to that shown in FIG. 8 obtained in each tensile test sample of the forged article manufactured under the conditions B. When these graphs are compared, it is understood with respect to the former forged article that all the mechanical properties of the 0.2% yield strength, the tensile strength and the elongation after fracture are higher in the tensile test sample having the tensile direction according with the plastic flow direction than in the tensile test sample having the tensile direction according with the forging direction. In contrast, with respect to the latter forged article, all the mechanical properties of the 0.2% yield strength, the tensile strength and the elongation after fracture are equivalent in the tensile test sample having the tensile direction according with the forging direction and the tensile test sample having the tensile direction according with the plastic flow direction.

[0084]FIGS. 10A and 10B respectively show the microstructures on the surfaces of the forged article manufactured under the conditions A and the forged article manufactured under the conditions B. It is understood from these drawings that the forged article manufactured under the conditions A has a microstructure in which the matrix of the liquid phase part is interspersed with the solid phase parts and the solid phase parts are aligned along the plastic flow direction while the forged article manufactured under the conditions B has a homogeneous microstructure. The anisotropy in the mechanical properties of the forged articles is probably derived from this difference in the microstructure. Also, the forged article manufactured under the conditions A has higher mechanical properties along the plastic flow direction than along the forging direction probably because interfacial flaking between the liquid phase part and the solid phase parts can be easily caused in the latter direction than in the former direction.

[0085] (Experiment 2)

[0086] <Experiment Method>

[0087] An injection molded material in the form of a metal plate was shaped by using the injection molding apparatus from an alloy B having a composition listed in Table 1. In this case, the temperature of the molten metal was controlled so as to attain a solid phase proportion of 5% in the resultant injection molded material, and the solid phase proportion was confirmed through image analysis of the surface of the resultant injection molded material. The alloy B used herein is AZ91D according to ASTM standards. Similarly, an injection molded material in the form of a metal plate is shaped from an alloy C having a composition listed in Table 1. In this case, the temperature of the molten metal was controlled so as to attain a solid phase proportion of 10% in the resultant injection molded material.

[0088] From each of the injection molded materials in the metal plate shape made from the alloys B and C, several articles for forging each in the form of a block with a width of 10 mm, a length of 35 mm and a thickness of 21 mm were cut out. The articles for forging cut out from the material of the alloy B were subjected to a pre-forging heat treatment at 410° C. for 16 hours and cooled, and those cut out from the material of the alloy C were subjected to a pre-forging heat treatment at 400° C. for 10 hours and cooled.

[0089] Then, each of the articles for forging having been subjected to the pre-forging heat treatment was forged, with its width direction constricted, until the thickness was reduced by half from 21 mm to 10.5 mm (namely, with a forging working rate of 50%), to give a forged article.

[0090] These forged articles made from the alloys B and C were then subjected to a post-forging heat treatment for 4 hours at a temperature of 170° C., 250° C., 300° C., 350° C. or 400° C. and cooled to give final forged articles. For comparison, some articles for forging were not subjected to the post-forging heat treatment to be directly used as final forged articles.

[0091] Then, a tensile test was carried out on the forged articles having been subjected to the post-forging heat treatment and the forged articles not having been subjected to the post-forging heat treatment.

[0092] With respect to each of the forged articles made from the alloys B and C and having been subjected to the post-forging heat treatment at 300° C., 350° C. or 400° C., the microstructure on the surface obtained after the tensile test was observed with a microscope. At this point, for comparison, the surface observation was performed on forged articles having been subjected to not pre- and post-forging heat treatments but the T6 heat treatment (in which the solution treatment was carried out at 410° C. for 16 hours and the ageing precipitation hardening treatment was carried out at 170° C. for 16 hours on the forged article made from the alloy B, and the solution treatment was carried out at 400° C. for 10 hours and the ageing precipitation hardening treatment was carried out at 175° C. for 16 hours on the forged article made from the alloy C).

[0093] <Experiment Results>

[0094]FIG. 11 shows the relationship between the temperature of the post-forging heat treatment and the 0.2% yield strength, the tensile strength and the elongation after fracture obtained in the forged articles made from the alloy B, and FIG. 12 shows the relationship corresponding to that of FIG. 11 obtained in the forged articles made from the alloy C. It is understood from FIGS. 11 and 12 that as the processing temperature is increased, the 0.2% yield strength is lowered, the tensile strength is slowly lowered and the elongation after fracture is increased in the forged articles made from both the alloys B and C. Also, when an article for forging is subjected to a heat treatment equivalent to the ageing precipitation hardening treatment (at 170° C. through 230° C.) of the T6 heat treatment, the elongation after fracture is smaller than in the case where an article for forging is not subjected to the heat treatment, but if the processing temperature is 250° C. or more, the elongation after fracture can be largely improved without largely lowering the 0.2% yield strength and the tensile strength.

[0095]FIGS. 13A through 13D show the microstructures of the forged articles made from the alloy B, and specifically, FIG. 13A shows that of a forged article having been subjected to the T6 heat treatment, FIG. 13B shows that of a forged article having been subjected to a heat treatment at 300° C., FIG. 13C shows that of a forged article having been subjected to a heat treatment at 350° C. and FIG. 13D shows that of a forged article having been subjected to a heat treatment at 400° C. FIGS. 14A through 14D show the microstructures of forged articles made from the alloy C respectively corresponding to FIGS. 13A through 13D. According to FIGS. 13A through 13D and 14A through 14D, it can be confirmed that crystal grains are coarse in those having been subjected to the T6 heat treatment (namely, those shown in FIGS. 13A and 14A) and also segregation (shown as black areas in the drawing) of a compound (Mg₁₇Al₁₂) is observed in the forged article made from the alloy B. In contrast, in those having been subjected to the post-forging heat treatments at a higher temperature for a shorter time than in the T6 heat treatment, when the processing temperature is 300° C. (namely, in those shown in FIGS. 13B and 14B), no clear grain boundary is observed and a compound is homogeneously precipitated. When the processing temperature is 350° C. (namely, in those shown in FIGS. 13C and 14C), fine grain boundaries are observed and the compound is homogenously precipitated. When the processing temperature is 400° C. (namely, in those shown in FIGS. 13D and 14D), crystal grains are coarse and the compound is homogeneously precipitated.

[0096] Judging from these results of the tensile test and the microstructure observation, the microstructure of a forged article obtained after a post-forging heat treatment is probably concerned in the ductility. Specifically, a structure that is not recrystallized is difficult to deform and hence has high tensile strength but low ductility. As crystal grains are more deformed through recrystallization, the ductility becomes higher, but when the crystal grains are too large, they are difficult to deform and hence are stiffened to degrade in the tensile strength and the ductility.

[0097] Accordingly, the post-forging heat treatment should be performed with a processing temperature properly selected so as to attain a microstructure where no crystal grains are observed in order to obtain a forged article with high tensile strength, and with a processing temperature properly selected so as to attain a microstructure where fine crystal grains are observed in order to obtain a forged article with high ductility.

[0098] (Experiment 3)

[0099] <Experiment Method>

[0100] In the same manner as in Experiment 2, several articles for forging each in the form of a block with a width of 10 mm, a length of 35 mm and a thickness of 21 mm were prepared from each of the alloys B and C. The articles made from the alloy B were subjected to a pre-forging heat treatment at 410° C. for 16 hours, and the articles made from the alloy C were subjected to a pre-forging heat treatment at 400° C. for 10 hours.

[0101] Each of the articles for forging having been subjected to the pre-forging heat treatment was forged, with its width direction constricted, until its thickness was reduced by half from 21 mm to 10.5 mm (namely, with the forging working rate of 50%), to give a forged article.

[0102] A post-forging heat treatment was carried out for 1 hour, 4 hours, 10 hours or 15 hours at a temperature of 300° C. on the forged articles made from the alloy B and at a temperature of 350° C. on the forged articles made from the alloy C, to give final forged articles.

[0103] A tensile test was carried out on each of the thus obtained forged articles.

[0104] <Experiment Results>

[0105]FIG. 15 shows the relationship between the processing time of the post-forging heat treatment and the 0.2% yield strength, the tensile strength and the elongation after fracture obtained in the forged articles made from the alloy B, and FIG. 16 shows the relationship corresponding to that of FIG. 15 obtained in the forged articles made from the alloy C. Data with the processing time of 0 (zero) correspond to data obtained in forged articles not subjected to the post-forging heat treatment in Experiment 2. According to FIGS. 15 and 16, it is understood that the 0.2% yield strength is comparatively largely lowered when the processing time is shorter than 1 hour but is slowly lowered when the processing time is longer in the forged articles made from both the alloys B and C. Also, the tensile strength is slightly improved when the processing time is shorter than 1 hour but is slowly lowered when the processing time is longer. On the other hand, the elongation after fracture is largely improved when the processing time is shorter than 1 hour but is substantially constant when the processing time is longer in the forged article made from the alloy B while it is largest when the processing time is 1 hour but is lowered when the processing time is longer in the forged article made from the alloy C. Accordingly, in the forged articles made from both the alloys B and C, the elongation after fracture can be largely improved during first 1 hour after the start of the heat treatment, and in the forged article made from the alloy C, the elongation after fracture can be largely improved when the processing time is 10 hours or less (preferably 5 hours or less).

INDUSTRIAL APPLICABILITY

[0106] As described so far, the present invention is useful for a manufacturing method for a plastic worked article in which an article for plastic working shaped by the semisolid molding is plastic worked after causing blisters by expanding gas cavities included in the article for plastic working through a pre-plastic working heat treatment. 

What is claimed is:
 1. A method for manufacturing a plastic worked article by plastic working an article for plastic working, comprising the steps of: shaping an article for plastic working by a semisolid molding in which molten metal including both a solid phase and a liquid phase is introduced into a die; performing a pre-plastic working heat treatment on the article for plastic working for causing blisters by expanding gas cavities included in the article for plastic working; and forming a plastic worked article by plastic working the article for plastic working after the pre-plastic working heat treatment, wherein the plastic working of the article for plastic working is performed in such a manner that a tensile stress applied direction at a tensile stress applied portion to which tensile stress is principally applied in actual use of the plastic worked article substantially accords with a plastic flow direction at a portion of the article for plastic working corresponding to the tensile stress applied portion.
 2. The method for manufacturing a plastic worked article of claim 1, wherein the pre-plastic working heat treatment is performed with a processing temperature in a range of 300° C. to 550° C.
 3. The method for manufacturing a plastic worked article of claim 2, wherein the pre-plastic working heat treatment is performed with a processing time of 1 hour or more.
 4. The method for manufacturing a plastic worked article of claim 3, wherein the pre-plastic working heat treatment is performed with a processing temperature in a range of 350° C. to 450° C. and a processing time of 20 hours or less.
 5. The method for manufacturing a plastic worked article of claim 4, wherein the pre-plastic working heat treatment is performed with a processing time in a range of 10 hours to 20 hours.
 6. The method for manufacturing a plastic worked article of claim 1, further comprising a step of performing a post-plastic working heat treatment on the plastic worked article obtained by the plastic working with a processing temperature in a range of 250° C. to 400° C. and a processing time in a range of 20 minutes to 10 hours.
 7. The method for manufacturing a plastic worked article of claim 1, wherein the article for plastic working is shaped by the semisolid molding using an injection molding apparatus equipped with a cylinder and a screw.
 8. The method for manufacturing a plastic worked article of claim 1, wherein a thick part is provided on the portion of the article for plastic working corresponding to the tensile stress applied portion, and the article for plastic working is shaped into a desired shape by the plastic working by allowing the thick part to flow in a direction substantially according with the tensile stress applied direction.
 9. The method for manufacturing a plastic worked article of claim 1, wherein the plastic worked article is a road wheel, and the plastic working of the article for plastic working is performed in such a manner that a direction substantially along an extending direction of a spoke of a wheel disk of the road wheel substantially accords with a plastic flow direction at a portion of the article for plastic working corresponding to the wheel disk.
 10. The method for manufacturing a plastic worked article of claim 1, wherein the plastic worked article is a road wheel, and the plastic working of the article for plastic working is performed in such a manner that a direction substantially along a width of a wheel rim of the road wheel substantially accords with a plastic flow direction at a portion of the article for plastic working corresponding to the wheel rim. 